A rubber/an acrylonitrile-styrene copolymer composition (hereinafter sometimes referred to as "ABS resin") is widely used because of its good workability and superior impact resistance. This ABS resin, however, has a disadvantage of being poor in heat resistance which is usually expressed in terms of a heat distortion temperature. In order to overcome the foregoing problem, various methods have been proposed, including:
(1) a method of substituting .alpha.-methylstyrene (hereinafter sometimes abbreviated to ".alpha.-MeSt") for styrene (hereinafter sometimes abbreviated to "St") of the components to be grafted onto rubber, i.e., St. and acrylonitrile (hereinafter sometimes abbreviated to "AN"); see U.S. Pat. No. 2,908,661;
(2) a method of blending an .alpha.-MeSt/AN copolymer with an ABS resin; see Japanese Patent Publication No. 18194/60;
(3) a two-stage grafting method in which St and AN are first grafted onto rubber and then .alpha.-MeSt and AN are grafted thereonto; see Japanese Patent Publication No. 13616/67;
(4) a method of blending an .alpha.-MeSt-methyl methacrylate (hereinafter sometimes abbreviated to "MMA")-AN copolymer to an ABS resin; see Japanese Patent Publication Nos. 18016/70, 33304/70 and 15902/69; and
(5) a composition comprising an .alpha.-MeSt-MMA-AN copolymer and a graft copolymer prepared by graft-polymerization of MMA, St and AN onto rubber; see Japanese Patent Publication No. 37415/71.
Substitution of .alpha.-MeSt and MMA for conventional resin components enables the artisan to increase heat resistance to a certain extent but not to a sufficient level. Thus, the resulting resin compositions are limited in their use. For this reason, it has been desired to develop an ABS resin having a more improved heat resistance.